System and method of transferring power in a gas turbine engine

ABSTRACT

A method of extracting work from a convertible gas turbine engine having a core flowpath and a bypass flowpath. The method comprises operating the convertible gas turbine engine at a first volumetric flow rate through the core flowpath and a second volumetric flow rate through the bypass flowpath to produce a first work output of the convertible gas turbine engine; extracting the first work output via an unshrouded fan and a shaft at a first fan to shaft extraction ratio; altering the second volumetric flowrate through the bypass flowpath while maintaining the first work output; and extracting the first work output via an unshrouded fan and a shaft at a second fan to shaft extraction ratio.

RELATED APPLICATIONS

This application is related to co-pending applications U.S. patentapplication Ser. No. 14/837,190 entitled “Splayed Inlet Guide Vanes”;U.S. patent application Ser. No. 14/837,302 entitled “Morphing Vane”;U.S. patent application Ser. No. 14/837,557 entitled “Propulsive ForceVectoring”; U.S. patent application Ser. No. 14/837,942 entitled “ASystem and Method for a Fluidic Barrier on the Low Pressure Side of aFan Blade”; U.S. patent application Ser. No. 14/837,079 entitled“Integrated Aircraft Propulsion System”; U.S. patent application Ser.No. 14/837,987 entitled “A System and Method for a Fluidic Barrier fromthe Upstream Splitter”; U.S. patent application Ser. No. 14/838,027entitled “A System and Method for a Fluidic Barrier with Vortices fromthe Upstream Splitter”; U.S. patent application Ser. No. 14/838,067entitled “A System and Method for a Fluidic Barrier from the LeadingEdge of a Fan Blade”; U.S. patent application Ser. No. 14/838,093entitled “Methods of Creating Fluidic Barriers in Turbine Engines”; U.S.patent application Ser. No. 14/837,031 entitled “Gas Turbine Enginehaving Radially-Split Inlet Guide Vanes”. The entirety of theseapplications are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to systems and methods used totransfer power. More specifically, the present disclosure is directed tosystems and methods which use articulating vanes to effect a transfer ofpower in a gas turbine engine.

BACKGROUND

Fluid propulsion devices achieve thrust by imparting momentum to a fluidcalled the propellant. An air-breathing engine, as the name implies,uses the atmosphere for most of its propellant. The gas turbine produceshigh-temperature gas which may be used either to generate power for apropeller, fan, generator or other mechanical apparatus or to developthrust directly by expansion and acceleration of the hot gas in anozzle. In any case, an air breathing engine continuously draws air fromthe atmosphere, compresses it, adds energy in the form of heat, and thenexpands it in order to convert the added energy to shaft work or jetkinetic energy. Thus, in addition to acting as propellant, the air actsas the working fluid in a thermodynamic process in which a fraction ofthe energy is made available for propulsive purposes or work.

Typically turbofan engines include at least two air streams. All airutilized by the engine initially passes through a fan, and then it issplit into the two air streams. The inner air stream is referred to ascore air and passes into the compressor portion of the engine, where itis compressed. This core air then is fed to the combustor portion of theengine where it is mixed with fuel and the fuel is combusted. Thecombustion gases then are expanded through the turbine portion of theengine, which extracts energy from the hot combustion gases, theextracted energy being used to run the compressor, the fan and otheraccessory systems. The remaining hot gases then flow into the exhaustportion of the engine, which may be used to produce thrust for forwardmotion to the aircraft.

The outer air flow stream bypasses the engine core and is pressurized bythe fan. Typically, no other work is done on the outer air flow streamwhich continues axially down the engine but outside the core. The bypassair flow stream also can be used to accomplish aircraft cooling by theintroduction of heat exchangers in the fan stream. Downstream of theturbine, the outer air flow stream is used to cool engine hardware inthe exhaust system. When additional thrust is required (demanded), someof the fan bypass air flow stream may be redirected to the augmenter(afterburner) where it is mixed with core flow and fuel to provide theadditional thrust to move the aircraft, in some applications

Many current and most future aircraft need efficient installedpropulsion system performance capabilities at diverse flight conditionsand over widely varying power settings for a variety of missions.Current turbofan engines are limited in their capabilities to supplythis type of mission adaptive performance, in great part due to thefundamental operating characteristics of their core systems which haslimited flexibility in load shifting between shaft and fan loading.

When defining a conventional engine cycle and configuration for a mixedmission application such as a mixed turbofan and turboshaft application,compromises have to be made in the selection of fan pressure ratio,bypass ratio, and overall pressure ratio to allow a reasonably sizedengine to operate effectively. In particular, the fan pressure ratio andrelated bypass ratio selection needed to obtain a reasonably sizedengine capable of developing the thrusts needed for combat maneuvers arenon-optimum for efficient low power flight where a significant portionof the engine output is transmitted to the shaft. In some applications,it is desired to reduce engine thrust in order to transfer more power toa shaft which drives a lift rotor, propeller, generator, or other deviceor system external to the turbofan engine.

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1A shows a generalorientation of a turbofan engine in a cut away view. In the turbofanengine shown the flow of the air is generally axial. The enginedirection along the axis is generally defined using, the terms“upstream” and “downstream” generally which refer to a position in a jetengine in relation to the ambient air inlet and the engine exhaust atthe back of the engine. For example, the inlet fan is upstream of thecombustion chamber. Likewise, the terms “fore” and “aft” generally referto a position in relation to the ambient air inlet and the engineexhaust nozzle. Additionally, outward/outboard and inward/inboard referto the radial direction. For example the bypass duct is outboard thecore duct. The ducts are generally circular and co-axial with eachother.

As ambient inlet airflow 12 enters inlet fan duct 14 of turbofan engine10, through the guide vanes 15 and passes by fan spinner 16, through fanrotor (fan blade) 42. The airflow 12 is split into primary (core) flowstream 28 and bypass flow stream 30 by upstream splitter 24 anddownstream splitter 25. In FIG. 2, the bypass flow stream 30 along withthe core/primary flow stream 28 is shown, the bypass stream 30 beingoutboard of the core stream 28. The inward portion of the bypass steam30 and the outward portion of the core streams are partially defined bythe splitters upstream of the compressor 26. The fan 42 has a pluralityof fan blades.

As shown in FIGS. 1A and 1B the fan blade 42 shown is rotating about theengine axis into the page, therefor the low pressure side of the blade42 is shown, the high pressure side being on the opposite side. Theprimary flow stream 28 flows through compressor 26 that compresses theair to a higher pressure. The compressed air typically passes through anoutlet guide vane to straighten the airflow and eliminate swirlingmotion or turbulence, a diffuser where air spreads out, and a compressormanifold to distribute the air in a smooth flow. The core flow stream 28is then mixed with fuel in combustion chamber 36 and the mixture isignited and burned. The resultant combustion products flow throughturbines 38 that extract energy from the combustion gases to turn fanrotor 42, compressor 26 and any shaft work by way of turbine shaft 40.The gases, passing exhaust cone, expand through an exhaust nozzle 43 toproduce thrust. Primary flow stream 28 leaves the engine at a highervelocity than when it entered. Bypass flow stream 30 flows through fanrotor 42, flows by bypass duct outer wall 27, an annular duct concentricwith the core engine, flows through fan discharge outlet and is expandedthrough an exhaust nozzle to produce additional thrust. Turbofan engine10 has a generally longitudinally extending centerline represented byengine axis 46.

A typical turbofan engine employs a two-shaft design, with ahigh-pressure turbine and the compressor 26 connected via a first shaftand a low-pressure turbine and the fan blade 42 connected via a secondshaft. In most designs the first and second shafts are concentricallylocated.

In most turbofan engines a significant portion of the engine's thrust isproduced by the rotation of fan blades 42 to create airflow in thebypass stream 30. However, as noted above in some applications it isdesirable to reduce an engine's thrust in order to transfer power toother systems, devices, or applications. Thus, an effective means isneeded to reduce a turbofan engine's thrust while maintaining overallpower produced by the core.

These and many other advantages of the present subject matter will bereadily apparent to one skilled in the art to which the inventionpertains from a perusal of the claims, the appended drawings, and thefollowing detailed description of preferred embodiments.

The present application discloses one or more of the features recited inthe appended claims and/or the following features which, alone or in anycombination, may comprise patentable subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The following will be apparent from elements of the figures, which areprovided for illustrative purposes and are not necessarily to scale.

FIGS. 1A and 1B are cutaway perspective views of typical turbofanengines.

FIG. 2 is an illustration of the bypass and core airflow paths in atypical turbofan engine.

FIGS. 3A and 3B are cutaway perspective views of a turbofan engine witha radially-split inlet guide vane in accordance with some embodiments ofthe present disclosure.

FIG. 4 is a profile view of a variable portion of a radially-split inletguide vane in accordance with some embodiments of the presentdisclosure.

FIG. 5 is a profile view of a variable portion of a radially-split inletguide vane in accordance with some embodiments of the presentdisclosure.

FIG. 6 is a cutaway perspective view of a turbofan engine with aradially-split inlet guide vane in accordance with some embodiments ofthe present disclosure.

FIG. 7 is a cutaway perspective view of a turbofan engine with aradially-split inlet guide vane in accordance with some embodiments ofthe present disclosure.

FIG. 8 is an isometric view of turbofan engine having radially-splitinlet guide vanes in accordance with some embodiments of the presentdisclosure.

FIG. 9 is an isometric view of turbofan engine having radially-splitinlet guide vanes in accordance with some embodiments of the presentdisclosure.

FIG. 10 is a cutaway perspective view of a turbofan engine having aradially-split inlet guide vane in accordance with some embodiments ofthe present disclosure.

FIG. 11 is a cutaway perspective view of a turbofan engine having aradially-split inlet guide vane forward of fan blade and aradially-split exit guide vane in accordance with some embodiments ofthe present disclosure.

FIG. 12 is a flow diagram of a method of transferring power betweenturboshaft and turbofan operations in accordance with some embodimentsof the present disclosure.

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the present disclosure is notintended to be limited to the particular forms disclosed. Rather, thepresent disclosure is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure asdefined by the appended claims.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to a number of illustrativeembodiments illustrated in the drawings and specific language will beused to describe the same.

This disclosure presents embodiments to overcome the aforementioneddeficiencies of conventional turbofan engines. More specifically, thisdisclosure is directed to systems and methods of transferring power ormaintaining a desired distribution between turboshaft and turbofan modesof operation in a gas turbine engine. In a method, air is admitted tothe core flowpath and bypass flowpath to establish a first work outputof a gas turbine engine. While maintaining core operating conditionsconstant, the flow rate of air to the bypass flowpath is altered totransfer work between the shaft and the fan. First work output ismaintained constant while work distribution is altered.

In some embodiments, the disclosed methods are executed in a gas turbineengine having an air inlet comprising a plurality of radially-splitinlet guide vanes having a first fixed portion to control airflow intothe engine core and a second variable portion to control airflow intothe engine bypass. In some embodiments, the disclosed methods areexecuted in a gas turbine engine having an air inlet comprising aplurality of radially-split inlet guide vanes having a first variableportion to control airflow into the engine core and a second variableportion to control airflow into the engine bypass. In some embodiments,the disclosed methods are executed in a gas turbine engine having aplurality of radially-split exit guide vanes having a first portion tocontrol airflow into the engine core and a second portion to controlairflow into the engine bypass, where first portion and second portionare either fixed or variable.

The disclosed method thus enables a turbofan engine to significantlyreduce its thrust output by reducing bypass airflow while maintaining aconstant overall engine power output by maintaining a constant volume ofcore airflow. Engine power can be transferred from thrust to otherapplications such as a lift fan, propeller, generator, or other deviceor system.

FIG. 3A is a cutaway perspective view of a turbofan engine 10 having aradially-split inlet guide vane 50. As described above, turbofan engine10 has an inlet fan duct 14 leading to a fan blade 42. A downstreamsplitter 25 divides air entering the turbofan engine 10 into a core flowstream 28 and a bypass flow stream 30. A single radially-split inletguide vane 50 is illustrated; a plurality of such vanes 50 are arrangedcircumferentially around the centerline axis for directing andcontrolling airflow entering turbofan engine 10.

Each vane 50 comprises a pair of lateral major surfaces forming aleading and a trailing edge. As illustrated in FIG. 3A, in someembodiments a radially-split inlet guide vane 50 comprises a firstportion 51 and second portion 52. In some embodiments the first portion51 is disposed radially inward from the second portion 52. The firstportion 51 directs air onto fan blade 42 and then into core flow stream28. In some embodiments the first portion 51 comprises a fixed blade.The second portion 52 is disposed radially outward from the firstportion 51, and directs air onto fan blade 42 and then into bypass flowstream 30.

In some embodiments fan blade 42 is one of a plurality of fan bladescomprising a single-stage adaptive fan which is operated in conjunctionwith fixed or variable inlet or exit guide vanes which load or unloadthe fan. When the fan utilizes the majority of the work output of thegas turbine engine then the engine is said to operate in turbofan mode,whereas when the majority of work output is driving the shaft the engineis said to operate in turboshaft mode.

FIG. 3A additionally illustrates an actuator 54 connected to secondportion 52. The actuator 54 is adapted to vary the position of secondportion 52, thus altering the geometry of the inlet fan duct 14. In someembodiments a stem 56 extends from its connection with the actuator 54through second portion 52 and into first portion 51, thus providing twoarticulating points for second portion 52. Stem 56 may provide the axisof articulation 53, which may be located at the aerodynamic center ofsecond portion 52 or may be located offset from the aerodynamic center.In some embodiments the actuator 54 is an actuation ring disposedtransverse to the direction of airflow 12 and radially outward from vane50. An actuation ring is connected to each second portion 52 of theplurality of radially-split inlet guide vanes 50 such that movement ofthe actuation ring causes articulation of each second portion 52.

FIG. 3B illustrates a second embodiment of radially-split inlet guidevane 50 having a lower protrusion 31 extending from second portion 52into first portion 51 and an upper protrusion 32 extending from secondportion 52 into a turbine casing 33. Upper protrusion 32 and lowerprotrusion 31 provide articulating points for second portion 52. An axisof articulation 53 is defined through upper protrusion 32 and lowerprotrusion 32. Actuator 54 is connected to second portion 52 via upperprotrusion 32. In some embodiments either upper protrusion 32 or lowerprotrusion 31 is omitted and second portion 52 has a single point ofarticulation.

In some embodiments such as those illustrated in FIGS. 3A and 3B secondportion 52 comprises a unitary member 55 which rotates about an axis ofarticulation 53. FIG. 4 is a profile view of such a second portion 52,illustrating the range of motion of a unitary member 55.

In some embodiments such as those illustrated in FIGS. 3A and 3B secondportion 52 can comprise a fixed strut 66 and a rotatable flap 67. FIG. 5is a profile view of one such embodiment which illustrates the range ofmotion of rotatable flap 67. As shown in FIG. 5, fixed strut 66 isdisposed upstream from rotatable flap 67, which articulates about axisof articulation 53.

FIG. 6 is a cutaway perspective view of a turbofan engine 10 having aradially-split inlet guide vane 50 of a different configuration thanthat illustrated in FIG. 3. Specifically, in FIG. 6 the radially-splitinlet guide van 50 comprises a unitary fixed portion 61 and a variableportion 62. The fixed portion 61 extends radially across the inlet fanduct 14, providing a fixed vane upstream from core flow stream 28 andthe fixed strut portion of the variable van upstream from bypass flowstream 30. Variable portion 62 is connected to actuator 54 andarticulates about an axis of articulation 53. In profile view, vane 50illustrated in FIG. 6 would appear similar to the second portion 52illustrated in FIG. 5, having a fixed strut (the fixed portion 61) androtatable flap (the variable portion 62).

FIG. 7 is a cutaway perspective view of a turbofan engine 10 having aradially-split inlet guide vane 50 of a different configuration thanthat illustrated in FIG. 3. Specifically, in FIG. 7 the radially-splitinlet guide vane 50 comprises a first portion 71 and second portion 72which are separated by an integral upstream splitter 24. As withprevious embodiments, first portion 71 is radially inward from secondportion 72 and is fixed. Second portion 72 is variable. In someembodiments second portion 72 is a unitary airfoil which rotates aboutan axis of articulation 53, while in other embodiments second portion 72comprises a fixed strut and rotatable flap. Upstream splitter 24 assiststhe radially-split inlet guide vane 50 and downstream splitter 25 individing inlet air into a bypass flow stream 30 and core flow stream 28.

FIGS. 8 and 9 are isometric views of a turbofan engine 10 having aplurality of radially-split inlet guide vanes 50. As both FIG. 8 andFIG. 9 show, a plurality of radially-split inlet guide vanes 50 extendradially outward from a centerline axis 81 and are radially contained bynacelle 82. FIG. 8 illustrates radially-split inlet guide vanes 50independent of an upstream splitter 24 and having fixed and variableportions configured as illustrated in FIG. 6. FIG. 9 illustratesradially-split inlet guide vanes 50 integral to an upstream splitter 24and having a fixed first portion 51 and variable second portion 52 asillustrated in FIG. 7.

FIG. 10 is a cutaway perspective view of a turbofan engine 10 having aradially-split inlet guide vane 50. A single radially-split inlet guidevane 50 is illustrated; a plurality of such vanes 50 are arrangedcircumferentially around the centerline axis for directing andcontrolling airflow entering turbofan engine 10.

As in the embodiment illustrated in FIG. 3A, the embodiment illustratedin FIG. 10 shows a radially-split inlet guide vane 50 comprising a firstportion 51 and second portion 52. Unlike the embodiment of FIG. 3Ahowever, both first portion 51 and second portion 52 are variableportions. The first portion 51 directs air onto fan blade 42 and theninto core flow stream 28. The second portion 52 is disposed radiallyoutward from the first portion 51, and directs air onto fan blade 42 andthen into bypass flow stream 30.

FIG. 10 illustrates a first portion actuator 101 connected to firstportion 51 and a second portion actuator 102 connected to second portion52. Each actuator 101, 102 is adapted to vary the position of itsrespective portion 51, 52, thus altering the geometry of the inlet fanduct 14. Variable first portion 51 and second portion 52 articulateabout an axis of articulation 53, which is illustrated as an axis commonto both first portion 51 and second portion 52 in FIG. 10. In someembodiments, first portion 51 has a different axis of articulation fromsecond portion 52. In some embodiments a stem (not shown) extendsthrough one or more of first portion 51 and second portion 52 to enablearticulation of the respective portion. In some embodiments the stem isconnected to an actuator such as first portion actuator 101 or secondportion actuator 102.

FIG. 11 is a cutaway perspective view of a turbofan engine 10 having aradially-split inlet guide vane 50 forward of fan blade 42 and aradially-split exit guide vane 110. Radially-split exit guide vane 110comprises a first exit guide vane portion 111 and a second exit guidevane portion 112, which can each be either fixed or variable. In someembodiments, first exit guide vane portion 111 and a second exit guidevane portion 112 are operated in conjunction with first portion 51 andsecond portion 52 to control airflow into the bypass stream 30 and corestream 28.

In some embodiments, core and bypass streams are split across the fanblade as described in U.S. patent application Ser. Nos. 14/837,942;14/837,987; 14/838,027; 14/838,067; and 14/838,093 which are hereinincorporated by reference.

FIG. 12 is a flow diagram of a method 1200 of transferring power betweenturboshaft and turbofan operations, and vice versa. The method 1200begins at step 1201 and proceeds simultaneously to steps 1203 and 1205.At step 1203 air is admitted into the core flowpath. In someembodiments, airflow to the core flowpath is controlled via an inwardportion of radially-split inlet guide vanes and/or radially-split exitguide vanes. In some embodiments the inward portion of radially-splitinlet guide vanes and/or radially-split exit guide vanes is variable,while in other embodiments the inward portion is fixed. Air is admittedinto the core flowpath at a first volumetric flow rate.

At step 1205 air is admitted into the bypass flowpath. In someembodiments, airflow to the core flowpath is controlled via an outwardportion of radially-split inlet guide vanes and/or radially-split exitguide vanes. In some embodiments the outward portion of radially-splitinlet guide vanes and/or radially-split exit guide vanes is variable,while in other embodiments the outward portion is fixed. Air can beadmitted into the bypass flowpath at a second volumetric flow rate,which may or may not be the same as the first volumetric flow rate.

Method 1200 then proceeds to step 1207, where the gas turbine engine isoperated at a first work output which is partially based on airflowthrough the core flowpath. At step 1209, a first distribution betweenthrust and shaft power (a thrust/shaft work ratio) is established in theoperating gas turbine engine. This first distribution can include fullthrust (zero shaft power), full shaft power (minimum thrust), or acontinuous range between full thrust and full shaft power in which thework output of the engine is distributed between thrust and shaft power.In embodiments having variable outward portions of radially-split inletguide vanes, the position of the variable portion can thus be describedas a full thrust position in which the variable portion provides maximumair flow to the bypass flowpath, a full shaft power position in whichthe variable portion is shut or partially shut to secure air flow orsignificantly reduce air flow to the bypass flowpath, and a continuousrange of positions between full thrust and full shaft power. In someembodiments the shaft of the gas turbine engine is connected to a liftfan, a propeller, a generator, or other device or system which requiresor receives shaft power.

Method 1200 then proceeds simultaneously to steps 1211 and 1213. At step1211 the flow rate of air admitted to the core flowpath is maintainedconstant, as are other core operating conditions. At step 1213 the flowrate of air admitted into the bypass flowpath is altered. In someembodiments, the flow rate to the bypass flowpath is altered byadjusting the variable portion of radially-split inlet guide vanes orradially-split exit guide vanes. In some embodiments, the position ofthe variable portion is adjusted by articulating a unitary airfoilaround an axis of articulation. In other embodiments, a variable portioncomprises a fixed strut and rotatable flap which is articulated aroundan axis of articulation. In some embodiments, an actuator or actuationring is used to adjust the position of the variable portion.

As an example, step 1213 could comprise articulating a unitary airfoilto reduce the effective surface area of inlet fan duct 14, resulting inless intake of inlet air into the bypass flowpath and subsequently inless thrust output from the gas turbine engine. Further, in someembodiments step 1213 comprises a first sub-step of coarsely adjustingthe flow rate of air admitted into the bypass flowpath by making a firstrelatively larger change in the position of the variable portion,followed by a second sub-step of finely adjusting the flow rate of airadmitted into the bypass flowpath by making a second relatively smallerchange in the position of the variable portion.

At step 1215 first work output of the gas turbine engine is maintainedconstant. At step 1217 a second thrust/shaft work ratio is established.The engine is operated at a second distribution between thrust and shaftpower. This second distribution can include full thrust (zero shaftpower), full shaft power (minimum thrust), or a continuous range betweenfull thrust and full shaft power in which the power output of the engineis distributed between thrust and shaft power.

Method 1200 ends at step 1219.

The disclosed systems and methods provide numerous advantages over theprior art. In applications requiring a gas turbine engine to operate inboth turbofan mode (producing thrust) and turboshaft mode (producingshaft power), the disclosed systems and methods allows for transitioningbetween these modes or balancing operation simultaneously between thesetwo modes. A single engine is thus capable of providing turboshaft powerto a rotorcraft, turboprop, generator, or similar shaft-powered deviceand then, without modifying operating conditions of the core engine,converts seamlessly and continuously to a high thrust turbofan whiledecreasing shaft extraction. Thrust can be significantly altered in anear-instantaneous manner and the engine can make a rapid transitionbetween turboshaft and turbofan modes of operation. In fact, the changesin thrust achieved by the disclosed method are more rapid than would beachievable using mechanical clutches between the turbine and the fanunit, and present advantages in applications requiring such rapidchanges in thrust, for example during a rapid egress of a militaryaircraft.

The disclosed systems and methods can be integrated into gas turbineengine designs which use a single stage fan or a two-stage fan, andwhich use any number of engine shafts. A further advantage is that fanblades of the turbofan engine are not required to be shrouded,segmented, or otherwise include devices which physically separateairflow into core and bypass flows. The use of unshrouded fan bladesresults in a simpler design which is more efficient to operate and lessexpensive to manufacture.

According to an aspect of the present disclosure, a method of extractingwork from a convertible gas turbine engine having a core flowpath and abypass flowpath comprises operating the convertible gas turbine engineat a first volumetric flow rate through the core flowpath and a secondvolumetric flow rate through the bypass flowpath to produce a first workoutput of the convertible gas turbine engine; extracting the first workoutput via an unshrouded fan and a shaft at a first fan to shaftextraction ratio; altering the second volumetric flowrate through thebypass flowpath while maintaining the first work output; and extractingthe first work output via an unshrouded fan and a shaft at a second fanto shaft extraction ratio.

According to an aspect of the present disclosure, a method of rapidlytransitioning between turboshaft and turbofan operations in aconvertible gas turbine engine comprising an unshrouded inlet fan and ashaft comprises establishing airflow through a set of fixed inlet guidevanes to an engine core flowpath to achieve maximum operating efficiencyof the convertible gas turbine engine; and transferring power betweenthe unshrouded inlet fan and the shaft while maintaining a constantpower output of the convertible gas turbine engine by altering thealignment of a plurality of variable inlet guide vanes forward of abypass flowpath.

According to an aspect of the present disclosure, a method oftransferring power in a convertible gas turbine engine having a coreflowpath through an air inlet, an unshrouded fan, a compressor, acombustor, and a turbine and a bypass flowpath through the air inlet andthe unshrouded fan comprises the steps of admitting a first volumetricflow rate of air into the core flowpath via a first portion of the airinlet comprising a plurality of fixed vanes, the first volumetric flowrate of air optimized for maximum power output of the convertible gasturbine engine; admitting a second volumetric flow rate of air into thebypass flowpath via a second portion of the air inlet comprising aplurality of variable vanes; and adjusting the incident angle of theplurality of variable vanes to alter the second volumetric flow rate ofair admitted into the bypass flowpath while maintaining the firstvolumetric flow rate of air admitted into the core flowpath constant.

Although examples are illustrated and described herein, embodiments arenevertheless not limited to the details shown, since variousmodifications and structural changes may be made therein by those ofordinary skill within the scope and range of equivalents of the claims.

What is claimed is:
 1. A method of rapidly transitioning betweenturboshaft and turbofan operations in a convertible gas turbine enginecomprising an unshrouded inlet fan, a plurality of variable inlet guidevanes axially aligned with and anchored to a set of fixed inlet guidevanes and a shaft, a method comprising: establishing airflow through theset of fixed inlet guide vanes to an engine core flowpath to achievemaximum operating efficiency of said convertible gas turbine engine; andtransferring power between said unshrouded inlet fan and said shaftwhile maintaining a constant power output of said convertible gasturbine engine by altering the alignment of the plurality of variableinlet guide vanes forward of a bypass flowpath.
 2. The method of claim 1wherein power is transferred from said unshrouded inlet fan to saidshaft by altering the alignment of said plurality of variable inletguide vanes to decrease airflow through the bypass flowpath.
 3. Themethod of claim 1 wherein power is transferred from said shaft to saidunshrouded inlet fan by altering the alignment of said plurality ofvariable inlet guide vanes to increase airflow through the bypassflowpath.
 4. The method of claim 1 wherein said alignment of saidplurality of variable inlet guide vanes is controlled by a controllervia an actuator.
 5. The method of claim 4 wherein said alignment of saidplurality of variable inlet guide vanes is adjusted based on apredetermined schedule of alignments.
 6. The method of claim 5 whereinsaid predetermined schedule of alignments includes a plurality ofalignments for said plurality of variable inlet guide vanes to transferloading of the convertible gas turbine engine between turboshaft andturbofan operations while maintaining steady overall loading of theconvertible gas turbine engine.
 7. The method of claim 1 wherein saidconvertible gas turbine engine is affixed to an aircraft and saidturbofan operations provide propulsive force to said aircraft.
 8. Themethod of claim 1 wherein said shaft is selectably coupled to a powertransfer shaft via a clutch.
 9. The method of claim 1 wherein said shaftis coupled to a power transfer shaft.